This invention relates to a method and apparatus for automatically focusing an electron beam in a scanning electron microscope or other apparatus having an electron optical system.
It is well known that in order to obtain a high resolution image when using a scanning electron microscope, etc., it is necessary to irradiate the specimen with an electron beam having a very fine spot diameter. In order to do this, in other words in order to focus the electron beam, the operator must adjust the focal length of the condenser lens in accordance with his own visual assessment of the microscope image, an operation which is extermely exacting and time consuming especially for a person untrained in this type of adjustment.
In recent times, concerted efforts have been made to automate the above focusing operation. One such proposed automatic focusing method and apparatus is disclosed in my U.S. Pat. No. 3,937,959, entitled "Method and Apparatus for Automatically Focusing", said method and apparatus being used in an electron beam scanning device incorporating an electron beam generating source, a condenser lens system for focusing the electron beam generated by said source on a specimen, a scanning means for scanning the electron over said specimen, a detecting means for detecting the signal emanating from said specimen resultant upon electron beam irradiation, and a display means for displaying the specimen image on a display device by using the output signal of said detecting means to modulate the scanning beam of the display device which is in synchronism with said scanning means.
The automatic focusing method disclosed in said U.S. Pat. No. 3,937,959 comprises the following steps:
(a) changing the condenser lens focal length stepwise in synchronism with said scanning means,
(b) converting the output of said detecting means into a signal indicative of and corresponding to the diameter of said electron beam,
(c) comparing two converting signals indicative of the beam diameter obtained in step (b) and,
(d) controlling the polarity and change width in step (a) in response to the result obtained in step (c) in order to minimize the beam diameter.
Moreover, in the automatic focusing method as disclosed in said patent, the amplitude and periodicity of the scanning means is kept constant during each automatic focusing operation. Such being the case, when carrying out high magnification image observation, steps (a) and (c) above may not be executed with sufficient accuracy. For example, if the amplitude and periodicity of the scanning means is kept constant at let's say an image magnification in the order of 1.times.10.sup.4 times (length of specimen area scanned by electron beam is 10.about.15.mu., CRT screen width is 10.about.15 cm) and the amount of electron beam defocus exceeds 10.mu., it will be impossible to automatically produce a signal indicative of the diameter of the electron beam. Accordingly, it is necessary to manually set the magnification of the scanning means to some low value and to then gradually increase the magnification upon completion of each automatic focusing operation until the desired magnification is reached.
Again, if the scanning speed is too high, the converted signal in step (b) will not accurately accord with the diameter of the electron beam. On the other hand, if the scanning speed is too low, the automatic focusing opertion becomes protracted. Accordingly, it is necessary to manually set the scanning speed of the scanning means appropriately prior to the commencement of the automatic focusing operation.
The object, therefore, of the subject invention is to provide an automatic focusing method in which the scanning means operates under optimum conditions, with respect to magnification and scanning speed, at all times.